If the watch has drilled lugs then this type of spring bar is ideal for use with a NATO.
While not everyone buys into the idea, it's suggested that the flanges on a spring bar can be tugged by a NATO and cause the bar to come loose. This type of spring bar should eliminate that possibility. If the watch doesn't have drilled lugs, you can still use this type of bar, but you will have to 'snip' the bar out with cutters of some sort.
For a leather strap the regular spring bars with flanges are fine to use.
Spring bars from Cousins are fine. I'va also had good experiences from
this seller on eBay.